The present invention relates to a semiconductor memory device, and more particularly, to a negative word line voltage generator for a semiconductor memory device.
A word line for a semiconductor memory device is activated and turns on a cell transistor during an active operation mode, and is inactivated and turns off the cell transistor during a precharge operation mode.
The cell transistor is composed of at least one NMOS transistor of which a gate is coupled to a word line. A negative voltage is used as an inactivation voltage to enhance a turn-off capacity of the NMOS transistor and other characteristics of the NMOS transistor. A negative word line voltage generator for a semiconductor memory device using ‘negative word line’ technique will be described below.
FIG. 1 is a conventional negative word line voltage generator for a semiconductor memory device.
Referring to FIG. 1, the conventional negative word line voltage generator includes a comparison unit 110, a pull-down driving unit 120, and a feedback unit 130.
The comparison unit 110 compares a division voltage VREF_D of a reference voltage VREF with a feedback voltage FB. The pull-down driving unit 120 pulls down a negative word line voltage VBBW in response to an output signal outputted from an output node NO of the comparison unit 110. The feedback unit 130 provides the feedback voltage FB corresponding to a voltage level of the negative word line voltage VBBW node to the comparison unit 110.
The following is a detailed description about the negative word line voltage generator for the semiconductor memory device.
The comparison unit 110 includes a differential amplifying circuit which receives the division voltage VREF_D of the reference voltage VREF and the feedback voltage FB. A bias block 111 of the differential amplifying circuit provides a sourcing current to a first input block 112 and a second input block 113. The first input block 112 receives the division voltage VREF_D into which the reference voltage VREF is divided in proportion to a resistance value of a first resistor R1 and a second resistor R2. The second input block 113 receives the feedback voltage FB corresponding to a voltage level of the negative word line voltage which is provided from the feedback unit 130. A loading block 114 is coupled between a negative voltage VBB terminal and the first and second input blocks 112 and 113, and is configured as a current mirror.
The pull-down driving unit 120 pulls down the negative word line voltage VBBW in response to the output signal of the comparison unit 110. The pull-down driving unit 120 is coupled between a negative word line voltage VBBW node and the negative voltage VBB terminal and is configured with an NMOS transistor MN3 which is controlled by the output signal of the comparison unit 110.
The feedback unit 130 provides the feedback voltage FB to the second input block 113. The feedback voltage FB is in proportion to a resistance value of a third resistor R3 and a fourth resistor R4 which are coupled in series between a power supply voltage VINT terminal and the negative word line voltage VBBW node.
In the negative word line voltage generator, when the negative word line voltage increases due to an over-load of the negative word line VBBW node, the feedback voltage FB increases, the voltage level of the output signal outputted from the output node NO of the comparison unit 110 increases, and the negative word line voltage is decreased by increasing a pull-down driving capability of the NMOS transistor MN3 of the pull-down driving unit 120 which is controlled by the output signal of the comparison unit 110.
However, due to the limitation of the driving capability of the NMOS transistor MN3, the negative word line voltage may not drop enough. That is, a variation of the negative word line voltage may not be controlled sufficiently.
Accordingly, as the negative word line voltage increases, a leakage current of a cell capacitor coupled to a cell transistor increases, and refresh characteristics and operation reliability of a semiconductor memory device are deteriorated.